Film carrier tapes, in which a number of wiring patterns are formed on a flexible insulating film, have been used for incorporating electronic devices in an apparatus. Examples of such a film carrier tape for mounting electronic devices thereon include TAB (tape automated bonding) tape, CSP (chip size package) tape, COF (chip on film) tape, BGA (ball grid array) tape and ASIC (application specific integrated circuit) tape. These film carrier tapes have inner leads to connect with terminals of electronic devices to be mounted thereon, and outer leads, which are in series with a respective inner lead, for establishing a connection with outside devices.
Some of these film carrier tapes are required to plate the outer leads with solder which is an alloy of tin and lead. Since solder contains lead which is a heavy metal, recently it has begun to be replaced with a tin-bismuth alloy under the recent global request of eliminating lead from use. An addition of bismuth lowers a melting point of tin-bismuth alloy, just like lead metal does in tin-lead (Pb) solder. Therefore, it had been thought that tin-bismuth alloy solder could be treated equally with tin-lead (Pb) solder by optimizing a content of bismuth in the alloy.
However, it was found that the melting point of the tin-bismuth alloy did not have a constant value in spite of the fact that the bismuth content in the alloy solder was always constant.
A study was made to better understand the cause of this melting point fluctuation, and it was found that bismuth content is not uniform in the alloy deposit. That is, bismuth content tends to be higher near the surface of a tin-bismuth alloy deposit, which makes the melting point of the alloy different between near the surface and deep inside of the alloy deposit. This non-uniform bismuth content distribution in the alloy deposit not only changes a melting point of the tin-bismuth alloy, but also may have an influence on the bonding properties of the leads having the alloy deposit thereon and lower a reliability of the film carrier tapes.
A study was made to investigate the cause of the non-uniformity of bismuth content in a tin-bismuth alloy solder deposit, and the following fact was found.
Tin-bismuth alloy deposit can be formed by electroplating as well as tin-lead solder deposit. Bismuth works to lower a melting point of tin-bismuth alloy as well as lead metal in solder. However, bismuth tends to substitute with tin by contact with tin in the plating solution which will not occur in case of lead.
In a plating apparatus for production of the film carrier tape, it is inevitably needed to provide a certain distance between an electroplating tank and a water-washing tank positioned downstream of the traveling direction of the film carrier tape due to structural reasons. Therefore, it is generally required about 10 seconds for the film carrier tape to enter the washing tank after exited from the electroplating tank.
The distance between the electroplating tank and the washing tank causes no problem in tin-lead solder plating where no substitution occurs between tin and lead metals. However, it will cause a problem in the plating of tin-bismuth alloy. That is, after the film carrier tape exits the electroplating tank, bismuth in the plating solution remaining on the tape will substitute with tin present near the surface of tin-bismuth alloy deposit. As a consequence, a surface of the alloy deposit will be modified because a composition in the vicinity of the surface is altered. Therefore, tin-bismuth alloy deposit will have the highest bismuth content near the surface where tin has been substituted with bismuth. Accordingly, the tin-bismuth alloy deposit will come to have different melting points in the deposit along its thickness direction.
This substitution between bismuth and tin could be prevented and as a result the tin-bismuth alloy deposit would have a uniform composition if the film carrier tape is washed in the water-washing tank immediately after carried out from the plating tank. However, the electroplating tank and the washing tank are separate devices so that no matter how close they are positioned to one another, the conveyance of the film carrier tape between them will require about 10 seconds with the plating solution remaining thereon. This is sufficient time period for the substitution to proceed between bismuth in the plating solution and tin in the alloy deposit.
The substitution between bismuth and tin must be prevented since it leads to fluctuation of a melting point of the tin-bismuth alloy. Also, it should be prevented to obtain stable properties for the film carrier tape including bonding properties.